Before reviewing the background art, it is useful to define certain terms. A therapeutic agent is one administered with the intent of changing in a beneficial manner some physiological function of the recipient. Therapeutic agents include drugs, proteins, hormones, enzymes, nucleic acids, peptides, steroids, growth factors, modulators of enzyme activity, modulators of receptor activity and vitamins. A diagnostic agent is one administered with the intent of illuminating some physiological function, while leaving physiological function unaffected. Diagnostic agents include radioactive isotopes for scintigraphy, electron dense labels for X-ray or computer tomography, and magnetic labels for magnetic resonance imaging.
Targeting is the modification of an agent so that after parenteral administration its uptake by a specific type or population of cells is increased, over that obtained with the unmodified agent.
Receptor mediated endocytosis (RME) is a process whereby molecules in the extracellular space bind to specific receptors on the cell surface and are internalized. Through the process known as RME, molecules injected into the vascular compartment are cleared (removed) from plasma.
Uptake by RME exhibits three general properties characteristic of ligand-receptor interactions generally: structural specificity, saturability and competition. Structural specificity is observed when a receptor can distinguish between closely related structures and only molecules with structures meeting the binding requirements of the receptor binding site are internalized. Often the receptors involved in RME are discovered by their ability to internalize or clear glycoproteins from circulation. Saturability is observed when the rate of an agent internalized via RME decreases with increasing concentrations of that agent. This results because, at high concentrations, the receptor approaches full occupancy or becomes saturated with ligand.
Competition is observed when the rate of internalization of an agent can be reduced by the presence of additional agents bearing a structural resemblance to the first agent. The additional agents compete for receptor binding sites and decrease the rate of internalization of the first agent. Saturability results when high concentrations of a single ligand compete for a limited number of receptor sites. Competition results when chemically different ligands bind to a limited number of receptor sites.
The uptake of substances by RME is a feature of normal, healthy cells. RME transport systems can be found on normal macrophages, hepatocytes, fibroblasts, and reticulocytes. RME enables cells to internalize a variety of macromolecules in plasma, such as asialoglycoproteins, low density lipoproteins, transferrin, and insulin. See Table 1 of Wileman et al. Blochem. J. 232:1-14; 1985 for a list of cells performing RME, which also contains a general review of RME. See also Table I of Menz, E. T. PCT WO 90/01295, filed Aug. 3, 1989. Conversion of normal cells to tumor cells (transformation), may be associated with an increase or decrease in the activity of receptors performing RME. In some cases, such as the RME performed by the asialoglycoprotein receptor of hepatocytes, transformation to cancerous hepatoma cells is associated with receptor loss. Stockeft and Becker, Cancer Res. 40:3632-3634; 1980. In many cases, like the antibody based targeting of drugs to tumor antigens, the antigens are increased on tumor cells and decreased on normal cells.
Polysaccharides like arabinogalactan, which interact with receptors involved in RME, are referred to as RME-type polysaccharides. Many common polysaccharides such as dextrans, dextrins, celluloses, hydroxyethylstarchs, heparins, starchs, dextran sulfates, carboxylmethylated dextran, and carboxymethyl cellulose do not interact with receptors involve in RME; they are referred to as non-RME polysaccharides.
With these definitions in hand, the relevant background art will be discussed. Non-RME type polysaccharides have been used in the synthesis of a variety of materials used as diagnostic or therapeutic agents. Jacobsen, T. EPO 0 186 947 B1; Schroder U.S. Pat. No. 4,501,726; Ranney, D. F. PCT WO 90/03190, filed Sep. 29, 1989; Groman U.S. Pat. No. 4,827,945; Groman U.S. Pat. No. 4,770,183. Ranney discloses the delivery of diagnostic agents (metal ions as magnetic resonance (MR) contrast agents), using a polymeric carrier which is directed to tumor cells. Ranney suggests, without detailed examples, that other therapeutic complexes may also be delivered using this method, for chemotherapeutic impact or to provide sensitization or augmentation for radiation treatment (Ranney, D. F. PCT WO 90/03190, filed Sep. 29, 1989, page 51). It is known that the RME-type polysaccharide arabinogalactan can be used to target certain diagnostic agents, particularly superparamagnetic iron oxide. Menz, E. T. PCT WO 90/01295, filed Aug. 3, 1988.
Therapeutic agents, on the other hand, have been typically targeted by liposomes and by glycoproteins. Normally after injection, liposomes are recognized as particulate matter and are subject to phagocytosis, which results in their concentration in the tissues of the reticuloendothelial system (RES). Materials within liposomes are then concentrated in tissues such as the liver, spleen and bone which comprise the RES. Surface-modified liposomes have been synthesized and can be cleared by RME, but the surface modification consisted of a coating of proteins or glycoproteins. Ranads, V. V., J. Clin. Pharmacol. 29:685-694; 1989; Dragsten et al. Biochim. Biophys. Acta 926:270-279; 1987.
Colloids and particles of differing sizes and compositions are recognized by the RES. For example, Imferon, a dextran coated colloidal ferric oxyhydroxide used for the treatment of anemia, is slowly cleared from the blood by the phagocytic activity of the macrophages of the RES. Henderson and Hillman, Blood 34:357-375; 1969. Radioactive diagnostic agents such the technicium sulfur colloids and many types of magnetic particles used as MR contrast agents are also cleared by the RES. For a discussion see Josephson, etal. Mag. Res. Imag. 8:637-646; 1990.
Glycoproteins internalized by RME have been used to target therapeutic agents. For a review of targeting strategies see Table II of Meijer and van der Sluijs, Pharm. Res. 6:105-118; 1989.